Abstract

Increasing grain yield is the most important subject of crop breeding. Here, we report the elevated expression of a conserved microRNA, OsmiR408, could positively regulate grain yield in rice by increasing panicle branches and grain number. We further showed that OsmiR408 regulates grain yield by downregulating its downstream target, OsUCL8, which is an uclacyanin (UCL) gene of the phytocyanin family. The knock down or knock out of OsUCL8 also increases grain yield, while the overexpression of OsUCL8 results in an opposite phenotype. Spatial and temporal expression analyses showed that OsUCL8 was highly expressed in pistils, young panicles, developing seeds and inflorescence meristem, and was nearly complementary to that of OsmiR408. Interestingly, the OsUCL8 protein was localized to the cytoplasm, distinctive from a majority of phytocyanins which localize to the plasma membrane. Further studies revealed that the cleavage of OsUCL8 by miR408 affects copper homeostasis in the plant cell, which in turn affects the abundance of plastocyanin proteins and photosynthesis in rice. This is the first report of the effects of miR408-OsUCL8 in regulating rice photosynthesis and grain yield. Our study further broadens the perspective of miRNAs and UCLs and provides important information for breeding high-yielded crops through genetic engineering